1. Field of the Invention
The present invention relates to a fixing unit and an electrophotographic image forming apparatus using the fixing unit, such as a copier, a printer, a facsimile machine, and a multi-task machine having plural such functions.
2. Description of the Related Art
As a mainstream technique for fixing toner onto a recording medium, a fixing unit using heat and pressure causes a pressure member to be brought into press-contact with a fixing member including a heat source and then causes a transfer sheet having an unfixed toner image thereon to pass through an area between the fixing member and the pressure member.
The fixing member is subjected to treatment, such as fluorine coating, for preventing its front surface from having toner attached. However, a so-called offset phenomenon, in which toner on a recording medium is slightly attached to the fixing member, may occur depending on environment and conditions on the recording medium. It is known that the offset toner remains on the fixing member and the pressure member contacting the fixing member and is finally reversely transferred onto the recording medium to cause disturbances in an image. To prevent this problem, the fixing member is provided with a cleaning member such as a cleaning web and a cleaning roller. The cleaning member is brought into press-contact with the fixing member and the pressure member to collect the offset toner.
Some of the fixing units like the one described above have the function of causing a pressure member to be moved relative to a fixing member to be brought into press-contact with the fixing member, reduce pressure between the pressure member and the fixing member, and separate the pressure member and the fixing member from each other. Further, it is known that this function is directed to prevent the degradation of the fixing member and the pressure member or used to separate, when the fixing unit is not used, the fixing member and the pressure member from each other so as not to put a load on the members and to cool only the pressure member. Further, other fixing units may have the function of establishing plural positional relationships between a pressure member and a fixing member such as a strong press-contact state and a weak press-contact state when the pressure member is brought into press-contact with the fixing member. It is known that this function is used to produce an optimum press-contact state between the pressure member and the fixing member depending on environment and conditions on a recording medium.
To perform a constant cleaning function at any positional relationship between the pressure member and the fixing member when the cleaning member is brought into contact with the movable pressure member, it is preferable that the cleaning member be brought into contact with the pressure member at a prescribed contact position, at a prescribed contact angle, and with a prescribed contact force. If a distance between a contact roller and the pressure member becomes large and the contact force of a cleaning web becomes too weak, the cleaning function cannot be satisfactorily performed, which results in disturbances in an image.
Further, if the distance between the contact roller and the pressure member becomes small and the contact force becomes too strong, there is a likelihood of a normal correction of toner with the cleaning web being hindered due to the degradation of the materials of the contact roller and the pressure member and an increase in a torque required for winding the cleaning web.
Further, there is no problem with the cleaning member being completely fixed with respect to the fixing unit if the moving amount of the pressure member is substantially small at the contact position of the cleaning member. However, if the pressure member is separated from the fixing member, its moving amount becomes large, which may cause cleaned-up toner to be scattered and retransferred onto a recording medium.
Further, if the cleaning member is a cleaning web, a positional relationship between the shafts of a cleaning web member that contacts the pressure member, a winding roller that collects a used cleaning web, and a supply roller that supplies a new cleaning web may be broken with the movement of the pressure member. This results in the cleaning web having slack, which may hinder a normal collection of a used cleaning web. To prevent this problem, it has been known to use a method for moving a cleaning web following the movement of a pressure member.
The above method, however, requires a user to manually remove a recording medium if a jam occurs after the recording medium has passed through a nip. On this occasion, the pressure member may be driven in a direction opposite to its original driving direction (i.e., rotation a roller and reverse traveling of a belt) depending on how the recording medium is removed. Consequently, the cleaning web excessively wound off the supply roller has great slack at a cleaning nip formed when the cleaning member contacts the pressure member. If the cleaning web has great slack to the extent that a normal winding operation of the cleaning web cannot be performed, a cleaning failure occurs, with the result that an abnormal image may be formed on a recording medium due to black spots, escaping of toner, or the like, and the feeding of sheets may be disabled since the slack web reaches a fixing nip.
As a countermeasure for this problem in terms of control, it has been known to use a method for feeding a next sheet after the cleaning web is wound by a certain amount following a jam. However, since this method causes a user to be on standby for a time required to wind the cleaning web by a certain amount and unnecessarily consumes the cleaning web, the service life of the cleaning web is reduced.
Further, as a countermeasure for this problem in terms of a mechanical configuration, it has been known to use a technique disclosed in Patent Document 3 for preventing a cleaning web from having slack. This technique has a mechanism for controlling the intensity of the contact force of a contact roller or the contact/separation thereof and is aimed at realizing an optimum cleaning function according to use situations. In the case of applying this mechanism as a countermeasure for the above problem, the contact roller separating from a pressure member with which the contact roller is brought into contact at the time of processing the jam described above eliminates concern about an excessive winding of the cleaning web from a supply roller even if the pressure member is driven in any direction. However, this method not only increases a manufacturing cost by an amount corresponding to the mechanism allowing for controlling the intensity of the contact force of the contact roller and the contact/separation thereof but also results in complicated control. Further, there is a case in which toner on a recording medium may be unfixed when a jam is caused in a fixing unit. Therefore, it is necessary to assume a case in which the toner enters a cleaning nip. Moreover, there is a case in which the toner is solidified between the cleaning web and the pressure member if the entered toner is in a semi-molten state to bond the cleaning web and the pressure member together. Therefore, even if the contact roller is separated from the pressure member, the cleaning web is pulled by a bonding part between the contact roller and the pressure member, which results in the cleaning web being wound off the supply roller.
Further, it has been known to use a mechanism for using a driving source for collecting a used cleaning web and supplying a new cleaning web. However, since a cleaning member is a consumable supply and required to be replaced, it is preferable that the number of replacement parts of the cleaning member be minimized in terms of manufacturing cost.
Further, since the fixing unit is at high temperature immediately after its operation, it can be assumed that a cleaning web unit is also at high temperature when being replaced. Accordingly, it is desirable that an operator not replace the cleaning web unit immediately after its operation since the operator may have a risk of being injured such as burning his/her hands. However, the operator is notified of the risk only by an alert message, and thus reliability is lacking. Therefore, it is preferable that fixing unit itself have a safety system as a hardware device.    Patent Document 1: JP-B2-4266027    Patent Document 2: JP-A-2009-037078    Patent Document 3: JP-A-2003-167465    Patent Document 4: JP-A-2000-321914    Patent Document 5: JP-A-2008-304714